1. Field of the Invention
The present invention relates to an arc tube which is useful as a light source of a vehicle head lamp or the like, and also to a method of producing such an arc tube.
2. Description of the Related Art
Because of their ability to provide illumination at high intensity, arc tubes are increasingly used today also as light sources of vehicle head lamps and other applications. Usually, an arc tube comprises: an arc tube body which is made of quartz glass, and in which a light emitting tube portion is formed; and a pair of electrode assemblies which are pinch-sealed to the ends of the light emitting tube portion, respectively.
As shown in FIG. 7, in each of electrode assemblies 116, a rod electrode 118 and a lead wire 120 are connected to each other via a metal foil 122. The electrode assembly is pinch-sealed to a quartz glass tube 2 which is to serve as an arc tube body, in a state where a tip end portion 118a of the rod electrode 118 is protruded into a discharge space 112.
The pinch seal is performed in the following manner. As shown in the figure, the electrode assembly 116 in the state where the lead wire 120 is held on the assembly is inserted to a predetermined position in the quartz glass tube 2 starting from the tip end portion 118a of the rod electrode 118.
In the electrode assembly 116, the rod electrode 118 is held in a cantilevered manner via the metal foil 122 which is thin. When a vibration load due to mechanical vibrations or the like acts on the assembly, therefore, the rod electrode 118 often resonates with the load to largely swing.
The degree of the insertion of the electrode assembly 116 into the quartz glass tube 2 is adjusted while monitoring the position of the tip end portion 118a of the rod electrode 118 by a camera 50. When the rod electrode 118 largely swings, however, an image taken by the camera is not clear, so that the position of the tip end portion 118a is hardly recognized. As a result, there arises a problem in that the electrode assembly 116 cannot be correctly inserted to the predetermined position in the quartz glass tube 2.
Usually, the lead wire 120 and the metal foil 122 are connected to each other by spot welding. A technique in which spot welding is employed as the process for the connection in order that the current density can be increased to enhance the weld strength has been proposed (see JP-B-63-40354).
In the case where spot welding is employed, however, the metal foil 122 is often bent in an end portion on the side of the lead wire 120 as shown in FIG. 8A. When the electrode assembly 116 in which the metal foil 122 is bent is inserted into a quartz glass, the tip end portion 118a of the rod electrode 118 sometimes abuts against an opening end 60a of an insertion guide pipe 60 as shown in FIG. 8B. In such a case, there arises a problem in that, as shown in FIG. 8C, the metal foil 122 is flexed so largely that, as shown in FIG. 8D, the electrode assembly 116 is inserted into the quartz glass tube 2 via the insertion guide pipe 60 while maintaining the state where the metal foil 122 is bent.